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<h2><a name="SECTION0017350000000000000000"></a><a name="event-objects"></a>
<br>
15.3.5 Event Objects 
</h2>

<p>
This is one of the simplest mechanisms for communication between
threads: one thread signals an event and other threads wait for it.

<p>
An event object manages an internal flag that can be set to true with
the <tt class="method">set()</tt> method and reset to false with the <tt class="method">clear()</tt>
method.  The <tt class="method">wait()</tt> method blocks until the flag is true.

<p>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><span class="typelabel">class</span>&nbsp;<tt id='l2h-3451' xml:id='l2h-3451' class="class">Event</tt></b>(</nobr></td>
  <td><var></var>)</td></tr></table></dt>
<dd>
The internal flag is initially false.
</dl>

<p>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-3452' xml:id='l2h-3452' class="method">isSet</tt></b>(</nobr></td>
  <td><var></var>)</td></tr></table></dt>
<dd>
Return true if and only if the internal flag is true.
</dl>

<p>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-3453' xml:id='l2h-3453' class="method">set</tt></b>(</nobr></td>
  <td><var></var>)</td></tr></table></dt>
<dd>
Set the internal flag to true.
All threads waiting for it to become true are awakened.
Threads that call <tt class="method">wait()</tt> once the flag is true will not block
at all.
</dl>

<p>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-3454' xml:id='l2h-3454' class="method">clear</tt></b>(</nobr></td>
  <td><var></var>)</td></tr></table></dt>
<dd>
Reset the internal flag to false.
Subsequently, threads calling <tt class="method">wait()</tt> will block until
<tt class="method">set()</tt> is called to set the internal flag to true again.
</dl>

<p>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-3455' xml:id='l2h-3455' class="method">wait</tt></b>(</nobr></td>
  <td><var></var><big>[</big><var>timeout</var><big>]</big><var></var>)</td></tr></table></dt>
<dd>
Block until the internal flag is true.
If the internal flag is true on entry, return immediately.  Otherwise,
block until another thread calls <tt class="method">set()</tt> to set the flag to
true, or until the optional timeout occurs.

<p>
When the timeout argument is present and not <code>None</code>, it should be a
floating point number specifying a timeout for the operation in
seconds (or fractions thereof).
</dl>

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